This invention relates to electrolytic process for producing aluminium, and more particularly to processes which employ lithium salts as one of the electrolytic bath additives.
The electrolytic baths in aluminium manufacturing processes comprise generally besides the basic components consisting of alumina (Al.sub.2 O.sub.3) dissolved in molten cryolite (Na.sub.3 AlF.sub.6), some additives which improve certain bath characteristics.
Among such additives, lithium salts, and particularly lithium carbonate (Li.sub.2 CO.sub.3), have recently found a widespread use due mainly to the fact that they enhance the electric conductivity of the cryolite baths and can lead accordingly to two alternative advantages. One advantage is the possibility of increasing the distance between electrodes at constant ohmic drop with consequent improvement in faradic yield of the respective cells. Alternatively, it is possible to increase the intensity of the current in electrolysis at constant distance between electrodes and constant ohmic drop, achieving in this case an increased cell productivity.
The main problem connected with the use of lithium carbonate in these processes consists in the necessity of reducing the specific consumptions of this bath additive owing to its high cost.
In the known processes the lithium carbonate was generally added to the electrolytic cells during their feeding with alumina, and this operation was usually a manual one consisting in strewing daily on the crust of the baths the required amount of the lithium salt and breaking up the crust to let into the baths this feed material. This method gave however rise to remarkable losses of lithium salt by vaporization and "dusting". The "dusting" phenomenon consisted in the reduction of the feed material in the form of a "dust" and its loss by dispersion, caused both by the mechanical action of breaking up the crust and the following chemical reaction which lithium carbonate had undergone on contacting the cryolitic bath: EQU (I) 3Li.sub.2 CO.sub.3 +2AlF.sub.3 6LiF+3CO.sub.2 +Al.sub.2 O.sub.3
the carbon dioxide evolved in the reaction entrained off remarkable amounts of lithium containing feed material in the form of a "dust". Since in the cells provided with hoods this feeding step is usually carried out with lifted cell hoods the so entrained quantity of lithium salt was non-recoverable and represented accordingly a net loss of material.
Besides these losses of lithium carbonate during the feeding step, further loss occurs by slow vaporization during operation of the cells and by slow impregnation in the carbon lining of the electrolytic cells. In quantitative terms, considering a desired concentration of lithium salt, expressed as LiF, in the cryolitic baths of about 3 to 5%, the above mentioned losses led to a specific consumption during operation of cells of about 5 to 10 Kg. Li.sub.2 CO.sub.3 /ton Al.
In order to diminish the specific consumptions of lithium carbonate in electrolytic cells of prebaked anode type by diminishing the "dusting" phenomenon during the feeding of Li.sub.2 CO.sub.3 to the cells a slow introduction of this additive in the electrolytic bath has been proposed such as by premixing the lithium carbonate with the alumina before their addition to the cells or by adding instead of lithium carbonate powder the more expensive lithium carbonate pellets. For cells of Soderberg anode type there has also been proposed to add powdered Li.sub.2 CO.sub.3 to the anodic paste.